Differential MEMS Sensor Interface for Common-Mode Noise Rejection
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Solution Overview
Problem
MEMS sensors with single-ended architectures struggle to distinguish between common-mode noise and actual signals due to poor power supply rejection ratio (PSRR) and electromagnetic compatibility (EMC) interference, leading to ineffective signal amplification.
Innovation Solution
Implementing a differential architecture for the MEMS interface circuit with a feedback control circuit that cancels common-mode signals by matching the capacitance of the MEMS transducer with an on-chip variable capacitor, allowing the feedback signal to be used to reject noise and interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended architecture is used for the MEMS interface circuit, then the device complexity is reduced, but the power supply rejection ratio (PSRR) and noise suppression capability deteriorate
Solution Approach 1:
The patent divides the interface circuit into differential signaling paths, separating the signal transmission into distinct differential pairs that can independently handle common-mode noise rejection while maintaining manageable circuit complexity through modular differential amplifier stages
Solution Approach 2:
The patent implements feedback control circuits that monitor the differential signals and actively adjust the circuit operation to maintain optimal PSRR performance, using the output signals to regulate the differential amplifier operation and compensate for power supply variations
2Device complexity
If a single-ended architecture is used for the MEMS interface circuit, then the circuit design is simplified, but the electromagnetic compatibility (EMC) interference suppression deteriorates
Solution Approach 1:
The patent segments the signal paths into differential pairs that are inherently more resistant to EMC interference, with each differential pair handling specific signal components and providing natural immunity to electromagnetic disturbances through balanced signaling
Solution Approach 2:
The patent employs asymmetric filtering and signal processing in the differential paths, where different frequency components and signal types are handled by specialized differential amplifier stages with tailored characteristics to optimize EMC performance
3Reliability
If a differential architecture is implemented with feedback control circuit, then the common-mode signal suppression is improved, but the device complexity increases
Solution Approach 1:
The patent uses feedback control circuits that take the differential output signals and feed them back to regulate the operation of the differential amplifiers, actively suppressing common-mode signals through closed-loop control while managing the increased complexity through efficient feedback topology
Solution Approach 2:
The patent combines multiple functions into integrated differential amplifier stages that perform amplification, common-mode rejection, and feedback control in unified circuit blocks, reducing the overall complexity despite the advanced functionality required for effective common-mode suppression
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively suppresses common-mode signals, improving signal-to-noise ratio and enhancing the ability to amplify actual signals while reducing noise and interference.
Implementation Method 1
the variable capacitor of the MEMS microphone changes its capacitance in dependence on a sound pressure impacting on the microphone
Implementation Method 2
matching the capacitance of the MEMS transducer with an on-chip variable capacitor
Data Source
Figure 1
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AI summary
A MEMS sensor (1) comprises a MEMS transducer (10) being coupled to a MEMS interface circuit (20). The MEMS interface circuit (20) comprises a bias voltage generator (100), a differential amplifier (200), a capacitor (300) and a feedback control circuit (400). The bias voltage generator (100) generates a bias voltage (Vbias) for operating the MEMS transducer. The variable capacitor (300) is connected to one of the input nodes (1200a) of the differential amplifier (200). At least one of the output nodes (A200a, A200b) of the differential amplifier is coupled to a base terminal (T110) of an output filter (110) of the bias voltage generator (100). Any disturbing signal from the bias voltage generator (100) is a common-mode signal that is divided equally on the input nodes (1200a, 1200b) of the differential amplifier (200) and is therefore rejected.